This invention relates to solid state electrochemical devices, methods of manufacturing thereof and applications for such devices, and more particularly, to apparatus and methods for preparing fuel cells.
Solid state electrochemical devices can be used as oxygen generators, separation devices, electrochemical reactors, and fuel cells. In a solid oxide fuel cell, a solid oxide electrolyte is used in combination with a compatible anode and a cathode material. Such solid oxide fuel cells generate electricity and heat by directly converting the chemical energy of a fuel (hydrogen, hydrocarbons) with an oxidant (O2, air) by means of an electrochemical process. Solid state electrochemical devices of the type addressed here make use of the property of certain solid state oxide electrolytes to support a current of oxygen anions, for example stabilized zirconia or related oxygen-ion conductors, which are effective at temperatures between 400 and 1000xc2x0 C. In the case of a solid oxide fuel cell, the fuel and oxidant are separated by the electrolyte membrane, with the cathode side in contact with the oxidant, and the anode side in contact with the fuel. Oxygen from the oxidant stream is reduced to O2xe2x88x92 anions at the cathode. These anions are transported through the solid electrolyte to the anode side of the cell. At the anode, the O2xe2x88x92 ions are reacted with the fuel stream thus releasing electrons to flow back to the cathode. A secondary device can be inserted into the circuit between the anode and cathode to draw useful work from the flow of electrons generated.
The fuel cell reaction is governed by the availability of reactants at the electrodes, oxidant at the porous cathode and fuel at the porous anode. The reaction also requires that the electrolyte material have sufficient ionic conductivity, and that a sufficient amount of both the anode and cathode materials be linked together in continuous conduction paths to support the required electronic current demands. The microstructure of the porous anode and cathode electrode materials and their associated number of electrochemically active three phase boundaries (xe2x80x9cTPBsxe2x80x9d) play an important role in governing the fuel cell electrode performance. Typically the conductivity of the electrolyte material increases with increasing operation temperature. Therefore, given a particular electrolyte material, the ohmic losses through the electrolyte membrane can only be reduced by either increasing the operating temperature of the cell or by reducing the thickness of the membrane.
Known solid oxide electrochemical devices all share the following features: 1) a manifold or area to introduce gases with high oxygen activity; 2) a manifold for gases at low oxygen activity; 3) an oxygen ion-conducting solid electrolyte separating the high oxygen activity gas from the low oxygen activity gas; 4) an electronically-conducting cathodic electrode to carry the cathode current; 5) an electronically-conducting anode electrode to carry the anode current; 6) a cathode-electrolyte-gas interface with high triple phase boundary (TPB) to enhance the electrochemical reaction rate; 7) an anode-lectrolyte-gas interface with high TPB. If a plurality of solid oxide fuel cell devices are connected in electrical series or in parallel, a suitable electrical interconnection may be required.
Known prior art for solid oxide fuel cells is reviewed well by Minh (xe2x80x9cFuel Cellsxe2x80x9d, Journal of the American Ceramic Society, March 1993 76(3) p. 563-558), disclosing various designs, including tubular fuel cells, monolithic fuel cells and planar fuel cells. Kendall (U.S. Pat. No. 5,827,620) teaches the extrusion of thin-walled electrolyte tubes. Co-fired monolithic fuel cells offering high power per unit volume are taught by Akerman et al (U.S. Pat. No. 4,476,198) and by Poeppel et al (U.S. Pat. No. 4,476,196). Kotchick et al (U.S. Pat. No. 4,913,982) and Minh (U.S. Pat. No. 5,788,788) teach the fabrication of monolithic fuel cells with a planar geometry by roll milling of unfired (or xe2x80x98greenxe2x80x99) ceramic tapes made from separate anode, electrolyte, and cathode layers. The roll milling process produces multiple-material tapes with a reduced thickness. The green assembly of Mihn and Kotchick et al is sintered at one time, so the several materials are co-fired.
As zirconia and the materials commonly used for the electrodes in solid oxide fuel cells are ceramics, techniques for assembly of such electrochemical devices have been adapted from the ceramic arts. These techniques include green body forming and hydroplastic or xe2x80x9cmudxe2x80x9d processing where a solvent such as water is introduced to powders to prepare a mudlike mixture suitable for extrusion. The mixture is formed into the desired shape by forcing it through an extrusion die which defines the desired shape. After extrusion, the extruded body or xe2x80x9cextrudatexe2x80x9d is heated or otherwise dried to remove the solvent. The xe2x80x9cmudxe2x80x9d nomenclature is an apt description until the solvent is removed. Moreover, the solid oxide extrudate is brittle after removal of the solvent. Further, production times are slow and geometric configuration and other design freedom is limited.
Continuing in the tradition of using various ceramic based technologies to improve the manufacture of solid state electrochemical devices, it is known to describe co-extrusion of a solid oxide fuel cell, using several hydroplastic materials (a mud-like mixture including anode in solvent, electrolyte in solvent, and cathode in solvent) forced through separate cylinders and then through a coextrusion die. However, mud-like materials are difficult to process, and it can be difficult to control the cross sectional geometry of the resulting electrochemical cell since these processes use the step of coextrusion to determine the geometry.
Popovic et al (U.S. Pat. No. 5,645,781) teaches a thermoplastic co-extrusion technique to produce a certain type of textured ceramic composite for mechanical applications. The method of Popovic"" et al. employs a controlled geometry feedrod having a macro-scale version of an arrangement of several ceramic powder-filled thermoplastic materials. The controlled geometry feedrod is forced through a heated die to produce finer diameter filament with a cross-section similar to the original feedrod. Popovic does not contemplate fabrication of electrochemical devices, nor does Popovic teach thermoplastic co-extrusion as a general technique of microfabrication for components suitable for solid oxide fuel cells.
The technique of microfabrication by thermoplastic co-extrusion (also referred to as xe2x80x9cMFCXxe2x80x9d), was recently developed and described by van Hoy et. al, xe2x80x9cMicrofabrication of Ceramics by Co-extrusionxe2x80x9d, Journal of the American Ceramic Society, January 1998, 81[1] p. 152-158); and by Crumm et al, xe2x80x9cFabrication of Microconfigured Multicomponent Ceramicsxe2x80x9d, Journal of the American Ceramic Society, April 1998, 81[4] p. 1053-1057); which make reference to formation of piezoelectric and other electromechanical devices.
It is desirable to obtain an electrochemical cell with large areas where the electrochemical reaction can take place (xe2x80x9cactive areasxe2x80x9d) per volume and excellent performance made from an inexpensive process. There is an ongoing need for fuel cell designs and fabrication techniques that can achieve these goals. The present invention provides such an approach and related advantages.
In accordance with a first aspect, a method for preparation of a solid state electrochemical device having a cathode, and anode and an electrolyte positioned between the cathode and the anode comprises the steps of forming a controlled geometry feedrod having a cross sectional area, having at least a first extrusion compound and a second extrusion compound, and co-extruding the controlled geometry feedrod through a reduction die at least once to form an extrudate having a desired reduction in the cross sectional area. The extrudate may be subsequently formed to change its shape, and the electrodes may comprise more than one discrete region.